This invention relates to a process for the preparation of lower alcohols by hydration of the corresponding olefins in a liquid phase.
For the preparation of alcohols by hydration of lower olefins a liquid phase method and a vapor phase method are known and an acidic catalyst is used in their methods. As compared with the vapor phase method the liquid phase method has advantages that alcohols with high conversion can be obtained starting from low-priced olefins of low purity and that utility consumption is small. However, since a strong acidic catalyst is used in the form of an aqueous solution under high temperature and high pressure, there are disadvantages that materials of an apparatus suffer corrosion markedly and the catalyst deteriorates owing to the products of corrosion. By way of example, in the vapor phase method phosphoric acid is used in the form supported on a carrier (U.S. Pat. No. 2,232,610). On the other hand the liquid phase method is disclosed in, for example, Canadian Pat. No. 845,202 and No. 867,797 where phosphoric acid is used in form of an aqueous solution. Under such circumstances that the aqueous solution of phosphoric acid is of high corrosiveness to materials of an apparatus and also, suitable anti-corrosion materials are not yet known, phosphoric acid has not been employed industrially as a catalyst in the liquid phase. According to the processes of the above Canadian patents, a reactor lined with ceramics is used for experiments in a laboratory scale.
Further, as disclosed in, for example, British Pat. No. 1,281,120, heteropolyacids are used as a catalyst in form of an aqueous solution wherein an alkaline substance for adjusting a pH is added to control the corrosion of apparatus materials and accordingly, the catalytic activity is sacrificed therefor.
The inventors found before that chromium compounds dissolved in an aqueous solution of phosphoric acid were effective anti-corrosive in the preparation of lower alcohols by hydration of lower olefins. The processes for effecting the hydration in an aqueous solution of phosphoric acid containing chromium oxyacids or oxyacid salts, or chromium salts and/or chromium complex compounds are disclosed in U.S. Pat. No. 4,060,541 and Japanese Patent Kokai No. 54-19904. These processes are characterized in that the corrosion inhibiting effect is in an elevated temperature with a small amount of anti-corrosive and that chromium compounds are good anti-corrosive even under a strong reducing atmosphere in which olefins and alcohols are present. However, there is a problem of precipitation of the anti-corrosive caused by iron ions which are formed through a slight corrosion and accumulated during the operation. Therefore the corrosion inhibiting effect is reduced and further, the precipitates bring about blocking of reactors and heat exchangers. For solving such problems methods for removing iron ions are proposed, but they result in complicated operation.